Bead portion of a tire

ABSTRACT

The invention is directed to a tire for mounting on the wheel rim of a vehicle. The tire comprises a pair of bead portions each reinforced by a bead hoop, at least one bead portion of the tire having an additional retention zone positioned axially inwards of and integral with the main bead portion, the retention zone having an axially inner tip or toe substantially inwards of the main bead portion and the construction of the bead portion and the additional retention zone between the bead hoop and tip providing substantial resistance to compressive forces therebetween. The portion between the bead hoop and the tip can include reinforcement and can comprise an elastomeric material having a hardness of 80 to 95 degrees I. R. H. The tip is formed from an elastomeric material having a set property at 70° C. of greater than 40% so that when the tip is fitted to a wheel rim having sharp teeth in the region contacted by the tip, the material of the tip engages the teeth. The tip may be provided by means of a strip of material positioned at the axially and radially innermost point of the tire toe. The tire can also comprise a chafer layer extending around the outer periphery of the bead in which case the strip of material providing the tip is positioned on the outside of the chafer.

This application is a divisional of U.S. application Ser. No.07/630,192, filed Dec. 19, 1990 now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a tire and wheel rim assembly and inparticular to tubeless pneumatic tire and wheel rim assemblies.

In conventional pneumatic tire and wheel rim assemblies the tire beadsare retained on their respective bead seats by means of the internal airpressure and commonly by the bead seats being tapered and/or the use ofa safety hump.

Deflation of the tire, however, or operation in a partially deflatedstate leaves the assembly in a relatively unsafe condition in which thetire beads may be moved from their respective bead seats by sidewaysforce. Such a condition may affect the control and safety of the vehicleand in the case of the commonest types of wheel rim having a well toallow the tire fitting, there is a grave danger of complete separationof the tire from its wheel rim and/or contact of the wheel rim flangewith the road surface. Both these conditions are dangerous.

Many suggestions have been made to deal with this problem including beadspacer rings and the tire and wheel rim assembly known as the TD tirewhich is disclosed in U K Patent Specification No. 1584553.

The latter TD type of construction uses a circumferentially extendinggroove in the wheel rim adjacent to the bead seat and a radially andaxially projecting toe on the tire engaging in the groove. Thisconstruction, however, requires a larger well in the wheel rim than in aconventional assembly which reduces the usable volume within the wheelfor brake space and also has a severe problem in compatibility in thatthe fitting of a tire without the necessary projecting toe to the TDtype of wheel rim gives an assembly which has a reduced bead retentionproperty to a standard tire on a standard wheel rim. This reduction inbead retention properties of a standard tire on the wheel rim so far hasmade it essential for the diameter of the TD type of tire and wheel rimto be made different from the normally accepted inch diameter tire andwheels to ensure this mis-match cannot occur in practice. This haslimited the adoption of the TD bead retention system.

Another prior art approach to the problem is disclosed in U K PatentSpecification 2026957 which uses a tire bead having a sector of the mainbead region, which is the axially and radially inner portion of anotherwise normal bead, constructed so that progressive rotation of thebead generates a substantially increasing compression in the bead andthus a continually increasing tension in the bead hoop. This can be doneeither by an increase in hardness around the sector or a progressiveincrease in dimension. The specification requires an additional frictionincreasing means between the tire contacting portion of the bead seatand the region of the bead seat which contacts the wheel rim in thenormally seated condition to ensure the necessary rotation withoutrelative slippage. Friction increasing means disclosed include knurlingor other mechanically formed grooving or teeth on the bead seat of thewheel rim and thus the disclosure teaches an assembly having a bead seaton the rim which is not suitable for a standard tire bead because of thespecially machined bead seat. The tire bead shape is not conducive tocorrect and secure seating or sealing of the assembly. Also reliance ismade on frictional grip once the bead has rotated beyond its initialrest position and the wheel rim cannot be used for a normal tire.

In each case of the prior art, whilst some retention may be provided,true compatibility with tires not of the required construction has notso far been achieved and in each case either the special tire cannot beused on a standard type rim, a standard tire cannot be used on thespecial rim or both.

SUMMARY OF THE INVENTION

Thus, it is an object of the present invention to provide a tire and awheel rim which when used together give fully effective bead retentionand yet allow for alternative fitments of standard tires or to standardrims without mis-match problems.

Accordingly, one aspect of the present invention provides a tire andwheel rim assembly for a vehicle, the tire comprising a pair of beadregions each reinforced by a bead hoop and the wheel rim comprising apair of bead seats characterised by one bead region of the tire havingan additional retention zone positioned axially inwards of and integralwith the main bead region, the retention zone having an axially innertip or toe substantially inwards of the main bead region and theconstruction of the bead between the bead hoop and tip providingsubstantial resistance to compressive forces therebetween and the wheelrim comprises axially inwards of and adjacent to one of the bead seats,which is that part of the bead seat contacted by a standard, acircumferentially extending tooth having a sharp edge inter-engagingonly the retention zone of the tyre at the tip such that when lateralforce is applied to the tire tread and internal inflation pressure islow, axially inward movement of the tip is resisted by tip-to-toothinter-engagement thus causing bead rotation about the tip and generationof compression force in those parts of the bead region between the tipand the hoop to provide bead retention at the bead seat.

Preferably the width of the additional retention zone is 5 to 30% of thewidth of the bead (J dimension). More preferably the width of theadditional zone is 15 to 25% of the J dimension. (In each case the Jdimension is the total width of the bead of the invention). Also thebead width of the tire from the heel point to the tip (which is the J.dimension) may be greater than the axial width of the bead seat of thewheel rim so that the tip is on a portion of the rim beyond the seat.

The bead and special tooth on the wheel rim are preferably provided atboth sides of the wheel rim so that both beads are retained. The beadseats may be on the radially outer surface of the wheel rim or on theradially inner surface to suit either the conventional tubelesspneumatic tire or the alternative system known as the CTS.

The wheel rim assembly and the wheel rim may have the bead seat of anyof the conventional tapered shapes including the shapes with more thanone angle of taper such as the contrepente wheel rim.

Another aspect provides a wheel rim for the tire and wheel rim assemblycomprising a pair of bead seats axially spaced apart and, adjacent toone of the bead seats and axially inwards thereof, a circumferentiallyextending tooth having a small radius edge or top.

Preferably the small radius is less than 2 mm to give sufficiently sharptooth. More preferably the radius is between 0.2 and 0.7 mm and inparticular between 0.2 and 0.3 mm or even 0.2 to 0.27 mm. In some casesa radius of 1.0 to 2.0 mm can be used. The final choice depends on thetoe material.

Three teeth may be provided although that number is not critical. Theimportant feature is to provide at least one tooth under the tipregardless of tolerances on tire and wheel rim dimensions.

The depth of the teeth may be in the range of 0.65 to 0.85 mm and thedistance from the top of one tooth to the top of the next adjacent toothis in the range of 1.2 to 1.8 mm, or 1.3 to 1.8 mm being 1.5 mm in oneembodiment.

The wheel rim immediately inboard of the bead seat may be a conventionalflat ledge or more preferably it comprises a bead retaining hump ofeither the round hump or flat hump type. In the case of humps the toothor teeth may be provided on the side of the hump nearer to the bead seatwhich ensure it is well clear of a conventional tire fitted to the rim.

The type of flange for the wheel rim is optional and any conventionalflange may be used in conjunction with the special bead of the presentinvention.

The resultant wheel rim provides the necessary tooth or teeth for thespecial tire of the present invention but also much more importantlyprovides for a standard tire bead of standard width with a bead seatwhich is entirely normal and which includes no special shapes orfinishes. Accordingly, a standard tire can be fitted to and run on therim of this invention without any problems and with the normal degree ofbead retention for the rim type be it flat ledge, hump, flat hump or thelike.

Preferably three distinct teeth are provided to give the tire beadengaging region and the region is positioned on a bead retaining humpprovided axially inwards of and adjacent to the inner edge of the beadseat. The retaining hump may be the conventional hump as in the ETRTOWheel Rim Standards and in that case the teeth are provided in theaxially outer slope of the hump such that the top of the axially mostoutward tooth is coincident with the standard rim profile. The remainingteeth are equi-spaced along a straight line sloping at 15° to thehorizontal so that their tops are slightly below the level of the saidstandard rim profile and the axially most inward groove is blended intothe standard rim profile. As a result, when a standard tyre is fitted tothe wheel rim, it is easily assembled in the same way as a normalassembly because it is effectively a standard rim. Once fully seated,however, a standard tire does not reach to the teeth but sits on thenormal bead seat.

Another aspect of the invention provides a tire having in at least onebead a circumferentially extending inextensible bead hoop and anadditional retention zone positioned axially inwards of and integralwith the main bead region, the retention zone having an axially innertip or toe substantially inwards of the main bead region, theconstruction of the bead between the bead hoop and the tip being such asto provide substantial form stiffness to indicated compression forcesbetween the hoop and tip and the tip of the tire being formed by amaterial having a compression set property at 70° Centigrade of greaterthan 40%. Set property is the property of an elastomeric material whichdetermines to what degree the material remains permanently deformed as aresult of the application of a compressive fore after the removal ofthat compressive force. Testing for the set property was carried out toASTM D395-69B.

The tip may be formed by a material having a hardness of 80 to 95degrees IRH. The material may also have a tear strength at roomtemperature of 30N/Test piece more. (Tested according to ASTM624 usingDie C.)

Preferably the width of the additional retention zone is 5 to 30% andmore preferable 15 to 25% of the width of the tire bead (J dimension).Also the bead width of the tire from the heel point to the tip (Jdimension) is greater than the axial width of the standard bead seatdimension for the tire of the size concerned so that the tip, whenfitted to a wheel rim contacts the rim inboard of the standard seat.

The tip material may be the material of the main part of the main beadregion and the additional retention zone as long as the requiredproperties are provided at the tip. More preferably, however, the tipmaterial is provided in a narrow tip zone extending axially outwardsfrom the tip along the radially inner face of the additional retentionzone for between 3 and 5 mm. The tip zone may be 1 to 3 mm in thickness,i.e. measured radially of the tire.

The tire bead may include a chafer ply extending around the bead surfaceat least from the heel point to a point on the axially inner face of thebead at the radial height of the bead core center. Preferably, the tipzone material is located outside said chafer so that the tip materialmay directly engage the teeth on a wheel rim.

The resultant tire has a bead which, whilst being wider than aconventional bead to provide the necessary inter-engagement with itsspecial region, has a standard shape of bead seat without radiallyprojecting toes and so the tire can be fitted to a standard wheel rimwithout difficulty.

Thus, the invention provides a tire which can be used on a standard rimwhen necessary with standard bead retention properties but which whenfitted to a special rim has greatly enhanced bead retention propertiesgiving a greater degree of safety. Furthermore, the special rim is alsoable to carry a standard tire with standard bead retention properties.The invention therefore provides a tire and wheel rim which allow forfull compatibility with standard tires and wheel rims known in the art.

The assembly may be used on any vehicle including cars, trucks,motor-cycles, aircraft etc.

The tire may be a cross ply, radial or any other known tire havingreinforced beads.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the present invention will now be described, by wayof example only, to show further aspects of the invention in conjuctionwith the attached diagrammatic drawings which are as follows:

FIG. 1 is a part cross section of the tire and wheel rim assemblyaccording to the present invention;

FIG. 2 is a part cross section of a tire and wheel rim assembly of theCTS type with bead retention according to the present invention;

FIG. 3 is an enlarged cross sectional view of one bead of the tire ofthe present invention;

FIG. 4 is an enlarged cross sectional view of the tire of the presentinvention on its rim;

FIG. 5 is a cross-sectional drawing showing the tire contacting profile,i.e. the radially outer profile, of a bead seat and flange at one sideof a wheel rim of the hump tire shown in FIGS. 1 and 4;

FIG. 6 is an enlarged view of part of FIG. 5;

FIG. 7 is an enlarged view of the same region of an alternative rimbased on a flat hump wheel rim;

FIG. 8 shows the tire of the present invention on a conventional roundhumped rim;

FIG. 9 shows a conventional tire on the wheel rim of the presentinvention;

FIG. 10 shows an alternative embodiment of the present inventionintended for a contrepente type wheel rim;

FIG. 11 shows an embodiment of the present invention for the CTS type oftire and wheel rim assembly;

FIG. 12 is a view of the bead rim of the invention to illustrate theoperation of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The tire shown in FIG. 1 comprises a tubeless tire carcass 1 fitted to awheel rim 2 having a central fitting well 3. The tire is reinforced by apair of bead hoops 4 and 5 and is also reinforced by conventionalcarcass and breaker assembly neither of which are shown as the mainstructure of the tire is not a feature of the present invention. Again,as is conventional, the wheel rim has two tapered bead seats 6 and 7which are tapered at 5 degrees and each bead seat has associatedtherewith an upright flange 8. The tire and the wheel rim haveco-operating engagement features 9 and 10 which will be described laterin relation to an enlarged view.

The embodiment shown in FIG. 2 is a CTS type tire and wheel rimassembly. This again has a tire carcass reinforced by a carcass ply andbreakers (not shown) and has two bead seats 6 and 7 which this time areon the radially inner surfaces of the wheel rim 2. The bead seats 6 and7 are tapered as is normal for this tire and wheel rim assembly but areprovided with additional features similar to those in FIG. 1 and whichagain will be described beneath.

FIG. 4 shows an enlarged view of one bead and its co-operating bead seatfor the tire of FIG. 1. The version for the CTS type tire of FIG. 2 isexactly the same in features although drawn on the inner face of thewheel rim and this design will not be described in detail.

The wheel rim, as shown in FIG. 5, is based on a standard wheel rimaccording to ETRTO dimensions of the round hump type but is modified inone region which will be described in detail below. This particularwheel rim is for a car tire and therefore comprises a tapered bead seat6 which is tapered at 5° to the axial direction of the wheel and anupwardly projecting rim flange 8 with a flared out region 8A. The wheelrim is of the nominal diameter for the tire concerned and has aconventional heel point HP as defined in the ETRTO Standards.

The bead seat 6 has a round hump 9 axially inboard of the seat and thishump 9 is positioned from the heel point HP exactly the same as in astandard rim. In order to show the shape of the standard rim, the region10 marked with a broken line is shown in FIG. 5 and in FIG. 6.

This particular rim is meant for a 14" tire and so it has a hump radiusof 8 mm and the dimension from the center of curvature of the hump tothe heel point HP is 19.8 mm.

In the region of the hump 10 shown by the broken line, however, fourgrooves are machined, or otherwise formed, so that three sharp edgedteeth 12 are left. These teeth 12 extend circumferentially of the wheel.Each tooth has a top radius R1 of 0.25 mm, a base radius R2 of 0.3 mmand a depth D1 of 0.75 mm. The distance D2 between the tops adjacentteeth is 1.5 mm. Thus, three sharp teeth 12 are provided on the wheelrim. The distance to the beginning of the first groove is the same asthe distance from the center rib of curvature of the hump 9 from theheel point and, therefore, D3 in this case is 19.8 mm.

The wheel rim shown in FIG. 7 is based on a wheel rim of the flat humptype which means that the original wheel profile is shown by numeral 11;but this too is modified in that the profile used comprises a series ofthree teeth 12, each of the same dimension as those in FIGS. 5 and 6.

In this case, however, the wheel rim is meant for a 15" diameter, tireand so that other dimensions of the wheel rim including the bead seatwidth are larger than for the smaller tire described above. Here thedistance from the top of the axially most outward tooth 12 to the heelpoint is 18.0 mm. Again, the top of this tooth is coincident with theflat hump profile whilst the other teeth 12 are below this profile.

The tire of the present invention comprises a bead hoop 5 of the 6×4creel type. Any normal bead may be used including one formed from asingle wire wound several times around a former so that only two endsoccur. A carcass ply 15 is wrapped in an axially outward directionaround the bead hoop 5 to form an edge turn-up 16 and a bead apex oftriangular cross section 17 is positioned upon the bead hoop 5 as shown.So far, the bead is of entirely conventional construction. However, thebead is made substantially wider than conventional beads for a tire ofthe same size and wider than the bead seat width B required to fitInternational Standard rims. This is achieved by providing a retentionzone 18 which is of triangular cross section and which provides anaxially inward extension of the bead to make it sufficiently wide sothat the axially inner edge of the bead engages the teeth 12. Theretention zone 18 is basically triangular in section having an axiallyinner surface 19 which merges to the line of the inner surface 20 of themain bead region as shown in FIG. 3. The retention zone 18 is integralwith the main bead region.

The width J2 of the resultant bead is substantially greater than thewidth J1 of the standard bead. For example, a 15" tire has a width J2 of20 mm compared with a standard bead width J1 of 17 mm.

The retention zone 18 has a profile similar to a standard tire providinga pointed tip 21 and a bead seat zone 22 which is a straightcontinuation of the main bead seat 23. In the region of the tip 21 a 1mm thick engagement strip 24 is provided. The strip has a width of 5 mmin the axial direction from the tip 21. This strip may extend a shortdistance up the axially inner surface 19 of the bead, but that is notnecessary.

The engagement strip 24 comprises rubber compound having a compressionset property at 70 degrees Centigrade of greater than 40%. This providesan engagement zone 24 which effectively creeps around the teeth 12 aswill be described below. A chafer strip 25 of cross woven nylon materialis positioned in the outer part of the bead extending around the toefrom beneath the bead hoop 5 to provide a skin on the toe of the overallbead. The cords of this strip are preferably laid at 45 degrees. Theengagement strip 24 is located on the outer surface of the chafer stip25.

The engagement strip 24 preferably has a hardness of 80 to 95 degreesIRH and a tear strength measured according to ASTM624 Die C at roomtemperature greater than 30 N/tp (Newtons per test piece) in order toprovide the required properties for use in engaging with the teeth 12.

For the remainder of the bead both the retention zone 18 and the mainbead between the bead core 5 and the tip 21 preferably comprise materialhaving a hardness greater than 80 degrees IRH. More preferably thehardness of this zone approaches 95 degrees IRH. This material hardnessmay be assisted by means of a filler material or other such means toprovide overall compression resistance.

Suitable compositions for the engagement strip 24 and the region of thebead between the tip 21 and the hoop 5 are as follows:

    ______________________________________                                        Engagement Strip Material                                                                     pphp                                                          ______________________________________                                        Polyoctenylene    40                                                          Natural rubber    60                                                          Zinc oxide        4                                                           Stearic acid      1                                                           Carbon black N326 80                                                          Reinforcing resin 5                                                           Tackifying resin  6                                                           HMT.sup.1         2                                                           NOBS.sup.2        1                                                           Enerflex 94.sup.3 5                                                           Sulphur           4                                                           ______________________________________                                         .sup.1 HMT is Hexamethylene Tetramine available from Montedison UK            .sup.2 NOBS is NOxydiethylene Benzthiazole 2Sulphenamido available from       Monsanto or Sanotocure MOR                                                    .sup.3 Enerflex 94 is a trademark of BP Oil Limited for an aromatic           process oil, with an equivalent being available from the Sun Oil Company.

    ______________________________________                                        Material for Region between Lip and Hoop                                                      pphp                                                          ______________________________________                                        Natural rubber    100                                                         High styrene resin                                                                              10                                                          N375 black        74                                                          Zinc oxide        4                                                           Stearic acid      2                                                           NCY               4                                                           Reinforcing resin 65%                                                                           9                                                           Aromatic oil      6                                                           Sulphur           4                                                           Hexa.sup.1          1.1                                                       NOBS                0.5                                                       ______________________________________                                    

The resultant materials have the following properties:

    ______________________________________                                        Engagement Strip Material                                                     ______________________________________                                        Hardness            89.3 Wallace D.L.                                                             I.R.H.D.                                                  Set at 70 degrees C.                                                                              55.2% ASTM                                                Tear at Room Temp   36 N/Test piece                                           ______________________________________                                         .sup.1 Hexa is a trademark good of Bayer UK Limited, the good being           composed of 80% HMT + 20% Ethylene Vinyl Acetate.                        

    ______________________________________                                        Material for Region between Lip and Hoop                                      ______________________________________                                        Modulus            6.7M pa @ 150% ext.                                        Resilience @ 50 degrees                                                                          39.5                                                       C. (Dunlop Tripsometer)                                                       Hardness (Wallace DL)                                                                            95 IRHD                                                    ______________________________________                                    

The tire and wheel rim of the present invention is shown assembled inFIG. 4 where it can clearly be seen that the dimensions of the tirecause the toe 21 to engage the teeth 12 with the material of theengagement strip 24 on the teeth 12. Thus, the tire is able to fullyseat against its flange 8 and on its bead seat 6 whilst the toe 21engages the teeth 12 by the material of the engagement strip 24 at leastpartially flowing around the teeth 12. It will be appreciated thatfitting of the tire to wheel rim is entirely conventional with airpressure blowing it home over the hump 9 which because it is of standardhump diameter provides no real resistance to tire fitting.

If the tire of the present invention is fitted to a conventional wheelrim either of the flat ledge type of the round, or flat hump type asshown in FIG. 8, the tire seats on the wheel rim in a very similar wayto on its own rim except that the engagement strip 24 and the tip 21simply lie upon the axially outer portion of the hump 9. In thiscondition the tire has very similar bead retention properties to astandard tire on the same rim as will be shown in the results quotedlater.

Another possible assembly is a standard tire bead 26 fitted to the wheelrim of the invention as shown in FIG. 9. Here the standard bead widthmeans that the conventional toe 27 is on the main bead seat 6 and thetoe does not reach the serrations or teeth 12. Thus as far as a standardtire is concerned for normal running it is seated on a rim which isentirely standard and when in the dislodgement mode a very similarperformance to standard tire on a standard rim is obtained as again willbe shown in the results following.

FIG. 10 shows an alternative construction using a contrepente rimprovided with a set of teeth 12 positioned again axially inwards of apoint where the toe 27 of a standard tire would be seated on thecontrepente bead seat 28. Details of the fit of the tire of theinvention to this rim are exactly the same as for the previousembodiment and so they will not be described here.

The tire and wheel rim shown in FIG. 2 of the CTS type has its one beadand bead seat shown enlarged in FIG. 11 and once again the bead seat 6has its normal taper to the point where the toe 28 of the standard CTStire reaches and then adjacent thereto but axially inwards of serrations12 which again engage the tip 21 of the tire according to the presentinvention. In this case the bead seat and its extension axially inwardsare shown flat, although a round hump or flat hump could be provided.

In use of the present invention the tire and wheel rim are seated asshown in FIG. 4 for all normal running. As mentioned previously fittingof the tire is entirely conventional using air pressure to blow thebeads home over the hump or flat ledge region, past the teeth 12 andonto the bead seat 6 with the bead contacting the flange 8. Due to theconstruction of the special tire however, its tip 21 is then inengagement with the teeth 12. For normal inflated running of the tirethe bead remains seated firmly on, its seat and against its flange 8.

If excessive side force is applied from the tire/ground interface or,when the tire is partially deflated which is much more common, whenthere is complete loss of air pressure within the tire then axiallyinward movement of the tire tread in a conventional tire and wheel rimassembly causes the bead to move axially inwards from its seat 6allowing air to leak around the bead and the tire to lose any remainingair pressure and much more importantly allowing the bead to move axiallyinwards where it can reach the well of the rim. This can result in thetire dismounting itself or the flange of the wheel rim contacting theroad, both these conditions being very dangerous. Furthermore, becausethe bead is not held at its seat 6 side force cannot be generated by thetire and vehicle handling is severely affected.

In the present invention, however, the tip of the tire engages aroundthe teeth 12 and so inward movement of the tip is resisted and, thus,the axially inward tread movement which applies a moment T to the beadzone causes rotation of the bead zone with the tip/toe engagement to theteeth 12 acting as a fulcrum. This brings the region 29 of the beadbetween the hoop 5 and the tip/tooth engagement point under increasingcompression which becomes greater and greater as the amount of rotationincreases. This compression force produces a reaction force in the beadhoop 5 which causes the hoop to be tensioned and this retains the beadaround the circumference of the tire firmly against the seat 6 so as toprevent bead dislodgement.

It should be noted that the moment T is generated by a force from thetread/road interface a distance away from the bead zone even in adeflated tire. In contrast, when a tire removal tool or bead breaker isplaced against the flange of the wheel rim and pushed against the tirebead the moment T is not generated and the tire may be dismounted in theconventional way.

To illustrate the effectiveness of the invention, tires in size 225/65VR 15 were manufactured having a bead width of 20 mm and using thecompounds described above for the engagement strip 24 and the zone ofthe bead between the hoop and the tip 21. The tires were tested on a rimaccording to the present invention. The tire was also tested on astandard rim of the same dimensions but without the teeth 12 and then astandard tire without the retention zone was tested on the wheel rim ofthe invention.

The test was in two parts and comprised a J turn test and a circlingtest, both of which are known in the industry for testing beadretention.

The J turn test comprises driving a vehicle with the test unit on theouter front wheel and with the vehicle loaded to its standard maximumload at a pre-set speed and applying as quickly as possible full turninglock whilst declutching. The vehicle then follows a J turn and comes toa halt. The test is repeated with progressive reductions of tirepressure of 0.2 bar until dislodgement occurs and then if necessary at 0pressure and higher speed increments.

The circling test comprises driving the vehicle at 60 kph in atangential direction into a 50 meters diameter circle and thenmaintaining the speed around the circle for one full turn. The vehicleis run in the fully laden condition and once again the tire pressuresare reduced stepwise in steps of 0.2 bar from standard pressure to 1.0bar and then in steps of 0.1 bar until dislodgement occurs. Thefollowing Table shows the results obtained.

                  TABLE                                                           ______________________________________                                                             J Turn Test                                                                             Circling Test                                                       Pressure/ Dislodgement                                   tire      Wheel      Speed     Pressure                                       ______________________________________                                        Invention Invention  0/40 mph  0.1 bar                                        Invention Standard   0.7 bar   0.8 bar                                                  flat hump                                                           Standard  Invention  0.9 bar   0.7 bar                                        tire                                                                          Standard  Standard   0.9 bar   1.0 bar                                        tire      hump                                                                ______________________________________                                    

As can clearly be seen the tire of the present invention has asubstantially improved bead retention property over all othercombinations. Furthermore, full compatibility of different tires ondifferent rims was achieved from those which would have been expectedfor a tire or wheel rim not according to the present invention.

For the tooth dimensions tests have shown that radii as small as 0.2 mmare successful with tires having the materials for the tire toe tip asdescribed. Radii smaller than 0.2 mm have been found to be sufficientlysharp to damage tires on dislodgement and sometimes on fitting and/orstripping, both of the special type and occasionally standard tires whenthey are fitted to the wheel rim.

With regard to the greatest radius, recent tests have shown for a cartire that 0.27 mm is a suitable maximum which provides a tooth stillsufficiently sharp to give good retention properties, although thefigures given in the original Application of up to 2 mm are still validdepending upon the properties selected for the tip material.

Similarly different tooth depths can be used although a minimum of 0.65mm and a maximum of 0.85 mm have been found to be particularly effectivefor a car wheel rim together with the range of 1.3 to 1.8 mm for thedistance from the top of one tooth to the top of the next adjacenttooth.

It must be noted that in each case the teeth are provided in the regionof the wheel rim which is not contacted by a standard tire if it isfitted to the wheel rim. Such a standard tire is indicated by the line Tin each of FIGS. 5, 6 and 7.

What is claimed is:
 1. A tire comprising in at least one main beadregion a circumferentially extending inextensible bead hoop and anadditional retention zone positioned axially inward of and integral withthe main bead region, the additional retention zone having an axiallyinner tip or toe substantially axially inward of the main bead region,the construction of the bead between the bead hoop and the tip beingsuch as to provide substantial form stiffness to withstand compressionforces between the hoop and tip and the tip of the tire being formed byan elastomeric material having a compression set property at 70°Centigrade of greater than 50% such that when the tire is fitted to awheel rim having sharp teeth in the region contacted by the tip, thematerial of the tip engages the teeth.
 2. The tire according to claim 1wherein the elastomeric material between the bead hoop and the tip has ahardness of 80 to 95 degrees IRH.
 3. The tire according to claim 1wherein the bead between the hoop and the tip includes a reinforcementmaterial to provide resistance to the compression force.
 4. The tireaccording to claim 1 further comprising a chafer layer extending aroundthe outer periphery of the bead from a point under the bead hoop, aroundthe tip and to a second point at least as high as the hoop centerline onthe interior of the tire to provide a skin reinforcement for the bead.5. The tire according to claim 1 further comprising at the tip of theretention zone which tip contacts a wheel rim when the tire is mountedon said wheel rim a tooth engagement strip comprising elastomericmaterial having a compression set property at 70° Centigrade of greaterthan 50%.
 6. The tire according to claim 1 wherein the elastomericmaterial of the tip has a tear strength at room temperature of 30 N/testpiece or more.
 7. The tire according to claim 1 wherein the tip isprovided by means of a strip of material having the said propertypositioned at the axially and radially innermost point of the tire toe.8. The tire according to claim 4 wherein the tip is provided by means ofa strip of material having the said property positioned outside thechafer layer and at the axially and radially innermost point of thetire.